How often do you change you main line?

I always find it easier to claim "old line" than cop to operator incompetence.

If I am in freshwater I'd flip it after two seasons. If I'm using it in salt (with the aforementioned contact with rox and barnacles, I'd remove the last 20 feet after each trip and retie. Braid shows the obvious damage (fraying and the like) by lightening up in color on the worn areas. Monitor the line and remove the worn area after each outing.

Two seasons tops, either flip if freshwater or discard if salt.

I’m still using the same braid on my freshwater bass tackles that I bought 10+ years ago

Wait?! Your spoda change it…….. before it gets too short from snags?!

...
4 years is prob maximum I'd go without replacing the line.
speaking of which, I gotta order some line!

I never change my braided line. Never had a problem. But if I were to fish for tuna or yellowtail in socal, I'd probably do it since those fish pull much harder.
I normally use Japanese braid (sunline or seaguar kureha etc) nowadays since they are much thinner for the same weight than the braid you'd find at a local tackle store.

I reverse my braid every 2 or 3 years. Sooner if it appears more worn than normal.